Exhaust scavenging in internal combustion engines is used to clear hot exhaust products from a combustion cylinder and recharge the cylinder with fresh intake air. Scavenging allows fresh air to flush out most of the residual hot exhaust gas from the clearance volume, and the resulting decrease in temperature reduces knock tendency, thus allowing greater spark advance which increases torque. Furthermore, the reduction in the amount of residual exhaust gas leaves more room for fresh air-fuel mixture which further increases torque. Scavenging is commonly used in the European market, but most turbocharged engines sold in the US market are calibrated to use much less scavenging, due to emissions concerns.
Traditional scavenging, or “blow through”, of fresh air is used when the intake manifold pressure is higher than exhaust manifold pressure. This is typical of turbocharged engines up to approximately 2000-2500 RPM. Turbocharged engines with tiVCT (Twin Independent Variable Camshaft Timing) can achieve low speed knock and torque benefits using scavenging. However, traditional scavenging can result in oxygenated fresh air passing through the exhaust valve and on to the exhaust aftertreatment system, resulting in increased emissions. The excess of oxygen in the exhaust is, in part, due to air flow used for traditional exhaust scavenging flowing through the exhaust valve during valve overlap. In some instances, this overlap allows intake air directly into the exhaust before the combustion stroke occurs. This unburned oxygen makes its way into the exhaust and can alter the chemistry of exhaust gas aftertreatment catalysts. In particular, increased oxygen lessens catalyst reducing ability thus decreasing the reduction of nitrogen oxides (NOx) prior to venting exhaust gases to the atmosphere. Furthermore, traditional scavenging relies on a pressure decrease from intake manifold to exhaust manifold in order to drive scavenging air flow through the cylinder. Under some operating conditions, this pressure differential is not present, decreasing the window of opportunity for exhaust scavenging.
The present disclosure describes a system and method for injection of a scavenging fluid, such as water or windshield washer fluid, directly into the cylinder, or via port injection, for vapor scavenging which can be achieved without excess oxygen to the exhaust system. The injected liquid evaporates quickly in the high temperature residual exhaust gas and/or when droplets impinge on hot metal surfaces in the combustion chamber. A relatively small amount of liquid can evaporate into a relatively large volume of vapor, thus displacing the residual exhaust gas and forcing it out the open exhaust valves. The use of vapor for scavenging reduces intake air released through the exhaust valve when compared to traditional scavenging that utilizes air flow across a pressure differential to displace residual exhaust gas. Furthermore, because the present disclosure relies on the expansion of liquid to vapor to displace the residual exhaust gas, vapor scavenging can occur even when intake manifold pressure is lower than exhaust manifold pressure. This method is adaptable to port injection or direct injection and may be compatible with various fuels, including diesel, gasoline, and ethanol as examples.
The disclosure describes a method for an engine comprising, injecting an amount of one of water and windshield washer fluid into a combustion cylinder based on a density and volume of residual exhaust gas. Port injection or direct injection scavenging with water or windshield washer fluid can reduce emissions concerns due to excessive oxygen in the exhaust aftertreatment system. Unlike traditional scavenging, the method of the present disclosure can also be used on engines without tiVCT, and even when intake manifold pressure is lower than exhaust pressure, such as at higher RPM, or at low RPM and medium-high load when knock is a concern. Furthermore, vapor scavenging may reduce the temperature of a combustion cylinder and thus reduce knock.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. Further, the inventors herein have recognized the disadvantages noted herein, and do not admit them as known.